S2.2.13 Expanded Octet of Electrons (Higher Level Only)

Some Atoms Can Form Molecules with Expanded Octets

Imagine you're solving a puzzle, and one of the pieces unexpectedly has extra slots to connect more pieces than usual.

This is similar to what happens when certain atoms form molecules with expanded octets—where they hold more than the typical eight electrons in their valence shell.

Expanded Octets and Their Connection to the Periodic Table

However, some elements in Period 3 and beyond (such as phosphorus, sulfur, and xenon) can exceed this limit. These atoms can accommodate more than eight electrons in their valence shell due to the availability of vacant d orbitals in addition to their s and p orbitals.

Why Only Period 3 and Beyond?

1. Electron Configuration:
   • Atoms in Period 3 and beyond have a principal quantum number $n\ge 3$, which means they have d orbitals in their valence shell (e.g., 3d for Period 3 elements).
   • These d orbitals can participate in bonding, allowing the atom to hold more than eight electrons.
2. Atomic Size:
   • Larger atoms, like sulfur and phosphorus, have more space in their valence shell to accommodate additional electrons, reducing electron-electron repulsion.

Visualizing Lewis Structures for Expanded Octets

To understand expanded octets, let’s explore how to draw Lewis structures for molecules where the central atom has more than four bonds (or electron domains).

Steps to Draw Lewis Structures for Expanded Octets:

1. Count Valence Electrons: Add up the valence electrons of all atoms in the molecule. For ions, adjust for the charge (add electrons for negative charges or subtract for positive charges).
2. Arrange Atoms: Place the least electronegative atom in the center.
3. Distribute Electrons: Connect atoms with single bonds and distribute the remaining electrons as lone pairs to satisfy the octet rule for peripheral atoms.
4. Expand the Octet: Place any leftover electrons on the central atom, allowing it to exceed eight electrons if it belongs to Period 3 or beyond.

Predicting Geometry Using the VSEPR Model

Once the Lewis structure is complete, the Valence Shell Electron Pair Repulsion (VSEPR)model helps us predict the molecule's geometry. The shape depends on the number of electron domains (bonding pairs and lone pairs) around the central atom.

Five Electron Domains: Trigonal Bipyramidal Geometry

When a molecule has five electron domains, they arrange themselves in a trigonal bipyramidal geometry to minimize repulsion. This geometry consists of:

• Three equatorial bonds at 120° angles in the same plane.
• Two axial bonds at 90° angles to the equatorial plane.

Molecular Geometries for Five Domains:

1. Trigonal Bipyramidal: All five domains are bonding (e.g., PF₅).
2. Seesaw: Four bonding domains and one lone pair (e.g., SF₄).
3. T-shaped: Three bonding domains and two lone pairs (e.g., ClF₃).
4. Linear: Two bonding domains and three lone pairs (e.g., I₃⁻).

Six Electron Domains: Octahedral Geometry

When a molecule has six electron domains, they adopt an octahedral geometry with 90° angles between all domains.

Molecular Geometries for Six Domains:

1. Octahedral: All six domains are bonding (e.g., SF₆).
2. Square Pyramidal: Five bonding domains and one lone pair (e.g., BrF₅).
3. Square Planar: Four bonding domains and two lone pairs (e.g., XeF₄).

Applications and Implications

The ability to expand the octet has far-reaching implications in chemistry:

1. Stability of Complex Molecules: Expanded octets allow the formation of stable molecules like SF₆ and PF₅, used in industrial applications (e.g., SF₆ as an electrical insulator).
2. Biological Relevance: Phosphorus and sulfur compounds with expanded octets are essential in biological molecules like DNA and proteins.
3. Environmental Concerns: Molecules like SF₆ are potent greenhouse gases with long atmospheric lifetimes, contributing to climate change.

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